The rate included in schedule A of § 50.05 shall not apply to that portion of a residence which becomes regularly used for commercial or manufacturing purposes. Where a portion of the customer's premises becomes regularly used for commercial or manufacturing purposes, the customer shall have the option of separating the wiring so that the residential portion of the premises can be served through a separate meter under the residential schedule applicable, and the commercial or manufacturing portion of the premises can be served through a separate meter under the applicable schedule for the service required. In the event the customer does not exercise the option of separating the wiring, the applicable commercial or power rate shall be applied to the total requirements of the customer.
('75 Code, § 6.14)